Aluminized steel

ABSTRACT

Steel base strip hot dip coated with substantially pure aluminum and adapted for porcelain enameling characterized by excellent reverse bend adherence. The interface iron-aluminum alloy must be less than about 0.45 ounces per square foot of sheet, and the free aluminum above the alloy layer must be in the range of 0.10 to 0.25 ounces per square foot of sheet.

United States Patent Dunbar et a1.

[ Dec. 16, 1975 1 ALUMINIZED STEEL [75] Inventors: Frank C. Dunbar,Monroe; Kasimir Oganowski, Middletown, both of Ohio [73] Assignee: ArmcoSteel Corporation,

Middletown, Ohio [22] Filed: Aug. 29, 1974 [21] Appl. No.: 501,765

Related U.S. Application Data [63] Continuation of Ser. No. 320,523,Jan. 2, 1973,

abandoned.

[52] U.S. Cl. 29/l96.2 [51] Int. Cl. B32B 15/18 [58] Field of Search29/191.6, 196.2

[56] References Cited UNITED STATES PATENTS 2,883,739 4/1959 Russell29/l96.2

3,058,206 10/1962 Mets ..29/l96.2

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-O. F. CrutchfieldAttorney, Agent, or Firm-Melville, Strasser, Foster & Hoffman [57]ABSTRACT Steel base strip hot dip coated with substantially purealuminum and adapted for porcelain enameling characterized by excellentreversebend adherence. The interface ir0n-a1uminum alloy must be lessthan about 0.45 ounces per square foot of sheet, and the free alu' minumabove the alloy layer must be in the range of 0.10 to 0.25 ounces persquare foot of sheet.

7 Claims, N0 D'rawings ALUMINIZED STEEL Cross Reference To RelatedApplication This is a continuation of application Ser. No. 320,523 filedJan. 2, 1973 and now abaondoned.

BACKGROUND OF THE INVENTION This invention relates to a coated steelsubstrate par ticularly adapted for porcelain enameling. Porcelainenamel (sometimes called vitrious enamel) is a fired-on opaque glassycoating applied to steel for use in a vari' ety of end products.

Most porcelain enameling to date has been carried out on plain steel oriron substrates. Since porcelain enameling is characterized by minutepinholes, the enameling is typically a multistage process, in that aplurality of layers of enamel will be fired on top of each other. Thus,the steel substrate is completely protected from corrosion.

It was later recognized that a steel substrate coated with acorrosion-resistant metal would be advantageous in porcelain enamelingprocesses. Under these circumstances, the thickness of the procelainenameling could be substantially reduced in that moisture or the likepassing through pinholes in the porcelain coating would be stopped bythe metallic coating. Similarly, the protective metallic coating makesit possible to porcelain enamel one side only for decorative purposes.

Further, the industry has recognized that aluminum is the preferredcoating metal for subsequent porcelain enameling, because it offersexcellent corrosion protection of the base steel and it is compatiblewith low temperature enambeling processes resulting in a more ductile,adherent product.

Most commercially available aluminum coatings today are applied by a hotdip process wherein a strand is subjected to various pretreatments andthen passed into a bath of molten aluminum. Typically, the moltenaluminum will contain approximately ten percent silicon alloyed with thealuminum. In order to porcelain enamel such stock, it is necessary tofirst prefire the aluminum coated steel. The precise influence mechanismof this prefiring is not fully known. It is, however, known thatattempts to porcelain enamel directly over a silicon bearing aluminumcoating usually results in a severe blistering. Prefiring at atemperature on the order of 1,000 F. is effective to eliminate theblistering problem.

It has also been known that blistering does not occur in the case of apure aluminum coating; that is, an aluminum coating containing nosilicon alloy. Therefore, a steel substrate coated with substantiallypure aluminum does not require prefiring in order to successfullyreceive a porcelain enamel coating.

However, the art has been unsuccessful in its attempts to produce asteel substrate having a pure alu minum coating which has good reversebend adherence and which will not delaminate from the steel base duringheating to a temperature of l,0O F. In other words, the application of aporcelain enamel coating requires heating to temperatures on the orderof l,O00 F. for 26 minutes. After this thermal cycling, the art hasfound that a steel substrate coated with substantially pure aluminum hasa tendency for gross coating delamination.

Keeping the above comments in mind, it is a primary object of thisinvention to provide an aluminum coated steel substrate particularlysuitable for porcelain enameling.

More specifically, it is an object of this invention to provide analuminized steel for porcelain enameling which is characterized byexcellent reverse bend adherence and climinates the coating delaminationhazard normally experienced during firing.

It is of course an object of this invention to provide an aluminizedsteel for porcelain enameling which does not require prefiring to avoidenamel blistering.

SUMMARY OF THE INVENTION Broadly considered, this invention relates to asteel base strip coated with substantially pure aluminum andparticularly adapted for porcelain enameling. The product ischaracterized by excellent reverse bend adherence and resistance tocoating delamination during firing of the porcelain enamel.

As just indicated, the coating metal must be substantially purealuminum, and preferably will be applied by a continuous hot dip coatingprocess.

The process is controlled so that the weight of the interface ironaluminum alloy will be less than about 0.45

ounces per square foot of coated sheet. The layer of free aluminum abovethe alloy layer must be continuous without alloy burn through, beyondthis limit, the free aluminum layer should be as thin as possible.Investigations have established that the free aluminum layer should bein the range of 0.10 to 0.25 ounces per square foot of coated sheet. Inany event, total coating weight must be less than about 0.60 ounces persquare foot of coated sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENT As previously indicated, thisinvention relates to an aluminized steel; that is, a steel base strandcoated with substantially pure aluminum. This aluminum coating may beapplied by any of the many, well known continuous hot dip coatingprocesses.

A typical hot dip coating process contemplates the rapid heating of abase steel strand in a non-oxidizing atmosphere so as to rapidly burnoil and the like from the surface of the strip. This is followed bycontinued heating in a reducing atmosphere, which is followed in turn bycontrolled cooling to bring the strip down to a temperature at or nearthe temperature of the molten aluminum. The strip is then passed intothe molten aluminum bath and withdrawn therefrom in a generally verticalpath of travel. The molten aluminum with drawn from the bath by themoving strand will be finished with a jet air knife in accordance withthe now well known techniques.

It is well known from prior experience that while the steel is incontact with molten aluminum, an interface iron aluminum alloy tends toform. On top of this interface alloy will be a layer of free orunalloyed aluminum. The essence of this invention appears to lie in thethickness of these layers; that is, the thickness of the interface alloylayer and the thickness of the free aluminum layer above the alloy.

It is important to this invention that the interface iron aluminum alloylayer be less than about 0.45 ounces per square foot of coated sheet,and preferably should be less than about 0.35 ounces per square foot ofcoated sheet. (In accordance with industry convention,

3 the figures in terms of ounces per square foot relate to both sides ofa coated strip.)

It is believed that the addition of silicon to the aluminum coatingmetal tends to retard the formation of this iron aluminum interfacealloy. Thus, microscopic (ixamination in cross section at 500X of aferrous base strand coated with an aluminum silicon alloy will disclosea rather smooth boundary line between the iron and the alloy, and willfurther show a very thin interface alloy level.

By comparison, microscopic examination at the same magnification of asection of ferrous strand coated with substantially pure aluminum willusually show a dogtooth or jagged, nonuniform boundary between the ironand the coating alloy layer. Similarly, the alloy layer is relativelymuch thicker.

It is very important to this invention that the free aluminum layerabove the interface alloy be continuous. in other words, the freealuminum must be thick enough to prevent contact between the alloy layerand the applied porcelain enamel coating. With this limitation, the freealuminum layer should be as thin as possible.

Empirical investigations have disclosed that the free aluminum layer ontop of the alloy layer should be on the order of 0.10 to 0.25 ounces persquare foot of coated sheet.

It will be apparent from the foregoing that the total coating weightshould be less than about 0.60 ounces per square foot.

A steel base strand with an aluminum coating having thesecharacteristics has been commercially produced on a coating lineconforming to that generally described earlier in this specification.The speed of the base strand through the coating metal bath variesbetween l2O and 450 feet per minute. This results in an immersion timein molten aluminum of from approximately 1.6 seconds to approximately 6seconds.

The molten aluminum bath was maintained at a temperature ofapproximately l,280 F., and the strip, after heating in the reducingatmosphere, was cooled to a temperature of approximately l,200 F. to1,250 F. prior to passage into the bath.

It is believed that the foregoing constitutes a full and completedisclosure of this invention, and no limitations are intended exceptinsofar as specifically set forth in the claims which follow.

The embodiments of the invention in which an exclusive property ofprivilege is claimed are defined as follows:

1. A steel base sheet hot dip coated with a coating consistingessentially of substantially pure aluminum, said aluminum coated sheethaving excellent reverse bend coating adherence and exhibiting nocoating delamination during heating to a temperature on the order of1,000F., said coated sheet being characterized by an interfaceiron-aluminum alloy layer having a weight less than 0.45 ounces persquare foot of coated sheet, and a continuous layer of unalloyedaluminum above said alloy layer, said unalloyed aluminum layer having aweight in the range of 0.10 to 0.25 ounces per square foot of coatedsheet, the total weight of said interface iron-aluminum alloy layer andsaid unalloyed aluminum layer being less than about 0.60 ounces persquare foot of coated sheet, whereby to provide an aluminized steel basesheet particularly suitable for enamel coating.

2. The product claimed in claim 1 wherein the weight of said alloy layeris less than 0.35 ounces per square foot of coated sheet.

3. A steel base sheet hot dip coated with a coating consistingessentially of substantially pure aluminum, said aluminum coated sheethaving excellent reverse bend coating adherence and exhibiting nocoating delamination during heating to a temperature on the order of1,000F., said coated sheet being characterized by an interfaceiron-aluminum alloy layer on each side of said sheet, and a continuouslayer of unalloyed aluminum above said alloy layer, the total weight ofsaid alloy layer and said unalloyed aluminum layer being less than 0.60ounces per square foot of coated sheet.

4. The product claimed in claim 3 wherein the weight of said unalloyedaluminum layer is less than 0.25 ounces per square foot of coated sheet.

5. The product claimed in claim 3 wherein the weight of said unalloyedaluminum layer is in the range of 0. 10 to 0.25 ounces per square footof coated sheet.

6. The product claimed in claim 3 wherein the weight of said alloy isless than 0.45 ounces per square foot of coated sheet.

7. The product claimed in claim 3 wherein the weight of said alloy layeris less than 0.35 ounces per square foot of coated sheet.

1. A STEEL BASE SHEET HOT DIP COATED WITH A COATING CONSISTINGESSENTIALLY OF SUBSTANTIALLY PURE ALUMINUM, SAID ALUMINUM COATED SHEETHAVING EXCELLENT REVERSE BEND COATING ADHERENCE AND EXHIBITING NOCOATING DELAMINATION DURING HEATING TO A TEMPERATURE ON THE ORDER OF1,000*F, SAID COATED SHEET BEING CHARACTERIZED BY AN INTERFACEIRON-ALUMINIUM ALLOY LAYER HAVING A WEIGHT LESS THAN 0.45 OUNCES PERSQUARE FOOT OF COATED SHEET, AND A CONTINUOUS LAYER OF UNALLOYEDALLUMINUM ABOVE SAID ALLOY LAYER, SAID UNALLOYED ALUMINUM LAYER HAVING AWEIGHT IN THE RANGE OF 0.10 TO 0.25 OUNCES PER SQUARE FOOT OF COATEDSHEET, THE TOTAL WEIGHT OF SAID INTERFACE IRON-ALUMINUM ALLOY LAYER ANDSAID UNALLOYED ALUMINUM LAYER BEING LESS THAN ABOUT 0.60 OUNCES PERSQUARE FOOT OF COATED SHEET, WHEREBY TO PROVIDE AN ALUMINIZED STEEL BASESHEET PARTICULARLY SUITABLE FOR ENAMEL COATING.
 2. The product claimedin claim 1 wherein the weight of said alloy layer is less than 0.35ounces per square foot of coated sheet.
 3. A steel base sheet hot dipcoated with a coating consisting essentially of substantially purealuminum, said aluminum coated sheet having excellent reverse bendcoating adherence and exhibiting no coating delamination during heatingto a temperature on the order of 1,000*F., said coated sheet beingcharacterized by an interface iron-aluminum alloy layer on each side ofsaid sheet, and a continuous layer of unalloyed aluminum above saidalloy layer, the total weight of said alloy layer and said unalloyedaluminum layer being less than 0.60 ounces per square foot of coatedsheet.
 4. The product claimed in claim 3 wherein the weight of saidunalloyed aluminum layer is less than 0.25 ounces per square foot ofcoated sheet.
 5. The product claimed in claim 3 wherein the weight ofsaid unalloyed aluminum layer is in the range of 0.10 to 0.25 ounces persquare foot of coated sheet.
 6. The product claimed in claim 3 whereinthe weight of said alloy is less than 0.45 ounces per square foot ofcoated shEet.
 7. The product claimed in claim 3 wherein the weight ofsaid alloy layer is less than 0.35 ounces per square foot of coatedsheet.